1. Field of the Invention
The present invention relates to an ultrasonic transmission/reception apparatus for use in, for example, making the diagnoses of the internal organs of living bodies by transmitting and receiving ultrasonic waves.
2. Description of a Related Art
In the field of medical treatment, a variety of image technologies have been developed in order to make diagnoses by observing the inner parts of objects to be inspected. Among them, ultrasonic imaging which acquires the internal information of the object by transmitting and receiving ultrasonic waves does not involve exposure to radiation, unlike the other medical image technologies such as X-ray photography and RI (Radio Isotope) scintigraphy. Therefore, ultrasonic imaging is utilized for a fetus diagnosis in an obstetrical region, and in extensive regions including a gynecological region, a circulatory system, a digestive system, etc., as the image technology of high safety.
In ultrasonic imaging, an image is generated by detecting ultrasonic waves which have been reflected at the boundary between substances of different acoustic impedances. Here, in general, an ultrasonic wave has the property of being more easily attenuated at a higher frequency. Therefore, an ultrasonic wave of comparatively high frequency (for example, 5 MHz or so) is employed in case of imaging the shallow part of the object, and an ultrasonic wave of comparatively low frequency (for example, 2 MHz or so) is employed in case of imaging the deep part of the object. Especially in a case where a boundary at which the difference of acoustic impedances is large exists as at a bone part, an ultrasonic wave of comparatively low frequency (for example, 0.5 MHz) is employed.
In case of the two-dimensional array of circular opening, the azimuth resolution ΔY of an ultrasonic beam is expressed in terms of a focal distance F, the wavelength λ of the ultrasonic wave and the diameter D of the opening, as follows:ΔY=1.22×F×λ/DSubject to the same size of the opening, as the wavelength λ of the ultrasonic wave is smaller, that is, as the frequency of the ultrasonic wave is higher, the value of ΔY becomes smaller, and the azimuth resolution is enhanced more. To the contrary, as the wavelength λ of the ultrasonic wave is larger, that is, as the frequency of the ultrasonic wave is lower, the value of ΔY becomes larger, and the azimuth resolution degrades more. On the other hand, the size of the opening is limited to a certain size in relation to the size of the object. Therefore, when an ultrasonic wave of low frequency is employed in order to image the deep part of the object, the azimuth resolution degrades, and hence, the minute structure of the deep part such as the interior of a bone cannot be imaged.
It is known that, when an object is irradiated with two ultrasonic waves of slightly different frequencies, vibration which has a frequency corresponding to the difference between the frequencies of the two ultrasonic waves is generated from a part irradiated with the ultrasonic waves. The vibration is also called “vibro-acoustic sound”. According to Samuel Calle, et al., “Application of Vibro-Acoustography to Bone Elasticity Imaging”, 2001 IEEE Ultrasonics Symposium, pp. 1601-1605, a mechanism for the generation of the vibro-acoustic sound is considered as follows: The vibro-acoustic sound will be generated by (1) the change of an impedance at the examined position of the object, or a radiation pressure based on the absorption and diffusion of the ultrasonic waves, or (2) such a physical phenomenon as the reflection of an ultrasonic beam which is generated by nonlinear interference.
Since such a vibro-acoustic sound has a frequency in an ultrasonic band of an audible range on the order of several kHz—several hundred kHz or so, a signal of sufficient intensity can be acquired even from the vibro-acoustic sound which has passed through a boundary having the large difference of acoustic impedances or has returned from the deep part of the object by way of example. Moreover, since the ultrasonic waves of high frequencies are employed in generating the vibro-acoustic sound, a high resolution can be realized by defining a scan region based on vibro-acoustic sounds. In the above thesis, therefore, it is described that the interior of a bone extracted from the object is imaged by employing the vibro-acoustic sounds. In Fatemi and Greenleaf, “Ultrasound-Stimulated Vibro-Acoustic Spectrography”, SCIENCE, Vol. 280, Apr. 3, 1998, pp. 82-85, it is described to image the minute structure of the deep part of the object. Further, in U.S. Pat. No. 5,903,516, it is disclosed to receive vibro-acoustic sounds in an audible range. Furthermore, in U.S. Pat. No. 5,991,239, it is disclosed that, in order to generate vibro-acoustic sounds, electronic scans are performed using a multi-ring annular array or a plurality of ultrasonic transducers.
However, continuous ultrasonic waves which are extraordinarily larger in the number of continuous waves than in case of ordinary ultrasonic imaging must be transmitted in order to generate vibro-acoustic sounds which have the number of continuous waves as required for generating an ultrasonic image. As a result, the distance resolution of detection data on the outer side of a bone part, namely, the shallow part of the object degrades drastically.